Earthquakes
Elastic Rebound Theory
Earthquakes occur when strain exceeds the strength of the rock and the rock fractures. The arrival of earthquakes waves is recorded by a seismograph. The amplitude of the P- wave displacement is used to determine the Richter magnitude.
Earthquake Waves Body waves move through the solid earth P-waves - longitudinal S-waves - transverse Surface waves - transverse
Recording Earthquakes Locating earthquake epicenter
Earthquake magnitude amount of energy released Determination of Richter Magnitude for an Earthquake M = log(x/t) + Y 1M ~ 10X Energy ~ X 2 1M ~ 100x energy However, energy increase when summed over the whole range of waves in a wave record is only 30x. Confusion time!
Earthquake intensity damage caused by earthquake Subsurface material Type of construction May not be directly related to earthquake magnitude
Rock mechanics experiments Let s break some rocks!
Triaxial load machine. a) cross-sectional sketch showing the pressure vessel, sample, and piston; b) photograph of machine.
Two deformed samples. L) induced fracture; R) saw-cut for friction experiments. There is a 5 mm-thick layer of gouge along the cut. Samples are 3.5" long and 2" in diameter.
Types of Faults Normal fault. Left side moved down relative to right side. Principal stress orientations
Right lateral strike-slip fault. Principle stress orientations Thrust fault. Block on left thrust up and over the block on the right. Principal stress orientations
Direction of movement Slickenslides show sense and direction of movement on a fault plane.
If water, or another fluid, occurs in a fault zone τ = μ(σ n P w ) = μs where P w = fluid pressure and (σ n P w ) = effective normal stress S. The famous beer can experiment an interesting way to spend an evening doing science. Fluids and earthquakes Rocky Mount Arsenal deep waste-disposal well and Denver earthquakes.
Orientation of present-day US principal stresses
Geology and Earthquakes in Eastern Massachusetts and New England
Large Earthquakes 1638-2012 All the known or suspected earthquakes of magnitude 5.0 or greater in New England and vicinity. There were damaging earthquakes centered in New England in 1638, 1727, 1755, 1904 and 1940. The largest earthquake on this plot was about M7.5 at Charlevoix, Quebec in 1663. 2010 (M5.0) Charlevoix Seismic Zone 1638 (M6.5?) 1904 (M5.9) 1940 (M5.5) 1727 (M5.6) 1755 (M6.2)
1727 Newburyport Earthquake The earthquake damaged chimneys and stone walls in Newbury, Massachusetts and nearby towns. It was felt to Philadelphia and Casco Bay, Maine. Estimated magnitude of this earthquake is 5.6.
Earthquake Hazards Primary ground motion and surface rupture Secondary fires, landslides, liquefaction, tsunamis
Primary effects - most earthquake damage is caused by differential movement of the land surface due to the passage of the transverse surface waves.
Secondary Effects Liquefaction Tsunami Landslide Fire Mud volcano
Seismic-risk map for the contiguous United States Boston is in the same seismic-risk zone as San Francisco
Peak Ground Acceleration (PGA) in m/s 2
Predicting Earthquakes 1) Recurrence intervals can only be applied to relatively frequent earthquakes 2) Seismic quiet zones an inactive area along a fault trace represents a region in which strain is accumulating a potential locus for failure 3) Changes in water level in wells as rocks begin to fail voids develop (dilatancy) and groundwater moves into the voids 4) Increase in frequency of small earthquakes precursor earthquakes 5) Strain measurements monitor accumulation of strain 6) Folklore
Dilatancy model Recurrence Interval Parkfield, CA Earthquake occurred in 2004
Engineering Solutions Design structure to be earthquake resistant 1) Anchor structure to bedrock 2) Strengthen against lateral movement 3) Secure façade to building frame 4) Decouple building mass from foundation Accept reality there is no such thing as an earthquake proof building. Earthquakes will occur, structures will fail, and there will be loss of human life. It is doubtful that earthquake prediction will ever have the immediacy to make evacuation a viable option.
Building 101
Earthquakes and the Earth s Interior
Seismic Discontinuities
Planet Earth the megascopic scale
Plate Boundaries and Earthquakes
A tectonic plate consists of the crust of the earth and the upper part of the mantle (the lithosphere). The lithosphere behaves as a rigid solid. The Moho is a seismic discontinuity within the lithosphere that marks the boundary between the crust and the mantle. The tectonic plate moves on the asthenosphere, a portion of the mantle that behaves plastically.
Four types of seismic activity characteristic of plate boundaries 1. Spreading ridges: shallow earthquakes, relatively low magnitude, occurring in lines 2. Transform faults: shallow focus, sometimes very powerful earthquakes 3. Continental collisions: shallow-deep focus in broad bands, can be very powerful 4. Subduction zones: deepest and most powerful earthquakes, some megathrust, some tsunami
Three types of plate boundaries (1) spreading centers, (2) subduction zones, and (3) strike-slip faults (plates are sliding past each other).
Spreading center mid-ocean ridge system and the East African rift system and rift valley
Subduction zone Earthquakes associated with oceanic trenches extend to great depths. This was a puzzle because for earthquakes to occur rocks must behave as elastic solids. At depths greater than 70 km rocks do not behave as elastic solids. Something else must be going on. The interpretation, backed by gravity and heat flow data, was that the seafloor was returning to the mantle of the earth. This process is referred to as subduction.
Strike-slip fault two plates sliding by each other
Convection in the Earth s Mantle Spreading occurs where mantle material rises and subduction occurs where mantle material descends.
Seismic velocity variations and mantle convection Seismic velocity variations can be correlated with differences in mantle density and inferred differences in temperature. Red = hot, blue = cold. Transfer of heat energy, by convection in the mantle, from the core of the earth to the surface is the driving force for plate tectonics.